Buoyancy control compensation systems as commonly used in SCUBA diving generally employ a collapsible bag carried by the diver and into which air is supplied to inflate the bag to increase the buoyancy of the diver. An inherent disadvantage of these prior art systems is the fact that unless air is supplied to or removed from the bag the buoyancy compensation provided by the bag changes as the depth of the diver changes.
In the U.S. Pat. No. 3,161,028 there is described a buoyancy compensation system employing a rigid compartment attached to the diver and to which water is supplied or removed in order to adjust the overall buoyancy of the diver. Since the volume of the compartment remains constant irrespective of changes in ambient pressure as the diver descends or ascends, the buoyancy of the compartment does not vary in relation to depth. An inherent danger in using this prior art system is that should the diver descend with the mouthpiece in his mouth, the increasing differential between the ambient pressure and the pressure in the compartment can cause serious harm to the diver. There is, therefore, a need for a constant volume buoyancy compensation system which is safe, which is relatively light in weight, and which can be manufactured and marketed at a reasonably low cost.